Isospecific Polymerization of Methyl Methacrylate by Intramolecular Rare‐Earth Metal Based Lewis Pairs†

Abstract

Rare-earth | Lewis pair | Tridentate ligand | Polymerization | Tacticity A series of cationic rare-earth aryloxide complexes, i.e., [LREOAr'] + [B(C 6 F 5) 4 ]-(L = CH 3 C(NAr)CHC(CH 3)(NCH(R)CH 2 PPh 2); RE = Y, Lu; Ar' =2,6-tBu 2-C 6 H 3 , 2,6-(PhCMe 2) 2-4-Me-C 6 H 2 ; Ar = 2,6-iPr 2-C 6 H 3 , 2,6-(Ph 2 CH) 2-4-iPr-C 6 H 2 ; R = H, CH 3 , iPr, Ph), were prepared and applied to the Lewis pair polymerization of methyl methacrylate (MMA). The stereoregularity of the resulting PMMA was significantly affected by the R substituent on the pendant arm of the tridentate NNP ligand, and was found to increase with increase in the steric hindrance of R. When using a Ph group as R, the Y complex produced a highly isotactic polymer with an mm value of 95% and a T g of 54.6 o C. In contrast, the steric hindrance of the Ar and Ar' groups had no effect on the tacticity of the resulting polymer, presumably because these two substituents were situated such that they pointed outward from the cyclic intermediates. Kinetics studies demonstrated that the polymerization was a first-order process with regard to the monomer concentration prior to catalyst deactivation. End group analysis indicated that the polymerization was accompanied by two possibly competing chain-termination side reactions that proceeded via intramolecular backbiting cyclization.

Tris(2,4-difluorophenyl)borane/Triisobutylphosphine Lewis Pair: A Thermostable and Air/Moisture-Tolerant Organic Catalyst for the Living Polymerization of Acrylates

“…More significantly, the P i Bu 3 → B­(2,4-F 2 C 6 H 3 ) 3 solid sample was also stable for up to 6 days under an open-air condition, as revealed by the NMR spectra (Figures S16–S18). These findings thus demonstrated the bench stability of this organic LP, in sharp contrast to highly sensitive metal-based LPs reported previously. ,− The appreciable air/moisture tolerance is presumably due to the strong interaction between the B­(2,4-F 2 C 6 H 3 ) 3 LA and the P i Bu 3 LB, which can effectively suppress the complexation of B­(2,4-F 2 C 6 H 3 ) 3 with H 2 O and therefore shut down the subsequent decomposition of borane via B–C bond protonolysis as well as irreversible deprotonation of H 2 O → B­(2,4-F 2 C 6 H 3 ) 3 by P i Bu 3 to form deactivated compounds (e.g., [H-P i Bu 3 ] + [HO-B­(2,4-F 2 C 6 H 3 ) 3 ] − ). It is worth pointing out that a delicate balance of LA–LB interaction has been met in the P i Bu 3 → B­(2,4-F 2 C 6 H 3 ) 3 CLA, which is strong enough to render its air/moisture tolerance but also is the weakest possible so that the CLA can readily convert into an FLP in the presence of a monomer to provide sufficient orthogonal reactivity for quantitative initiation.…”

“…Next, the thermal stability and air/moisture tolerance of the P i Bu 3 → B(2,4-F 2 C 6 H 3 ) 3 CLA were then evaluated. Heating a P i Bu 3 → B(2,4-F 2 C 6 H 3 ) 3 benzene-d 6 solution at 80 °C and monitoring its variation by the NMR spectra at different time intervals revealed virtually the same 1 H, 19 F, and 31 P NMR spectra up to 24 h (Figures S10−S12), indicative of the excellent thermal stability of this adduct. In addition, when using "wet" benzene-d 6 as the solvent, the NMR spectra of the P i Bu 3 → B(2,4-F 2 C 6 H 3 ) 3 sample were essentially unchanged within 24 h (Figures S13−S15).…”

“…Over the past 10 years or so, several robust LP catalysts have been developed (Scheme , top), including Al­(C 6 F 5 ) 3 /NHC [or P t Bu 3 , N -heterocyclic olefin (NHO)], ,− highly interacting AlR 3 /phosphine CLAs, MeAl­(BHT) 2 /NHO (or NHC, imidazolin-2-ylidenamino-substituted phosphine), − Al/P-bridged LPs, and rare earth metal-based LPs. − However, efficient and controlled/living LPPs of polar vinyl monomers reported to date all required the utilization of metal-based LAs with high oxophilicity, which are highly sensitive to air/moisture and, in some cases, to heating and shock [e.g., Al­(C 6 F 5 ) 3 ]. − On the other hand, the current LP catalysts only showed good control over molecular weight (MW) at room temperature (RT) or below. − To make LPP a truly green/sustainable and user-friendly polymerization methodology, it is highly desirable to construct a more tolerant metal-free LP catalyst that exhibits a controlled and effective polymerization behavior over a broad temperature range. Nevertheless, only a handful of LP organic catalysts were reported so far, which unfortunately promoted low active or ill-controlled polymerization. − Herein, with a rational design of structure, a new robust organic LP based on tris­(2,4-difluorophenyl)­borane [B­(2,4-F 2 C 6 H 3 ) 3 ; Scheme , bottom] LA and triisobutylphosphine (P i Bu 3 ) LB is developed in this work, which possesses good thermal stability and appreciable air/moisture tolerance.…”

Phosphonium ylide/organoaluminum-based Lewis pairs for the highly efficient living/controlled polymerization of alkyl (meth)acrylates

“…[21][22][23][24][25][26][27] It should be noted that only a few types of LBs have been discovered for this process. Currently, N-heterocyclic carbenes (NHCs), 24,28 N-heterocyclic olefins (NHOs), [21][22][23]29 and organophosphorus(III) bases (including alkyl phosphines, 30,31 imidazolin-2-ylidenamino substituted phosphines (IAPs), [32][33][34] and pyridinylidenaminophosphines (PyAPs) 35 ) have been reported (Fig. 1a), and are usually employed with organoaluminum compounds, such as AlR n (BHT) 3−n (R = Me, Et, iBu).…”

Rare-Earth Aryloxide/Ylide-Functionalized Phosphine Frustrated Lewis Pairs for the Polymerization of 4-Vinylpyridine and Its Derivatives

“…Since then, LPP has attracted a great deal of attention as an emerging polymerization technique. − In 2016, Rieger et al reported that a highly interacting Et 3 Al/PMe 3 Lewis pair enabled the polymerization of 4VP, albeit with a low activity . We were interested in replacing main-group-derived Lewis acids with rare-earth (RE) elements, which resulted in a typical FLP-like reactivity for RE complexes. − In our previous work, we developed several intra- − and intermolecular , RE-based Lewis pairs that were active for the LPP of methyl methacrylate (MMA) and its analogues. However, the intermolecular combination of a RE triaryloxide with organic phosphine, for example, PEt 3 , which was highly active toward acrylates polymerization, showed negligible activity for 4VP polymerization.…”